Travelling wave tube



May 2, 1961 J. P. F. NALOT TRAVELLING WAVE TUBE 2 Sheets-Sheet 1 FiledApril 4, 1957 INVENTOR JEAN PAUL FERNAND NALOT ATTORNEY May 2, 1961 J.\P. F. NALOT TRAVELLING WAVE TUBE 2 Sheets-Sheet 2 Filed April 4, 1957INVENTOR JEAN PAUL FERNAND NALOT ATTORNEY TRAVELLING WAVE TUBE Jean PaulFemand Nalot, Paris, France, assignor to Compagnie Generale deTelegraphic sans Fil, Paris, France Filed Apr. 4, 1957, Ser. No. 650,630Claims priority, application France Apr. 25, 1956 l 11 Claims. ci.sis-3.5

The present invention relates to travelling wave tubes, and moreparticularly relates to delay lines which may be utilized in these typesof tubes. 7

It is well known that heating of the delay line during the operation ofa travelling wave tube poses grave problems in the use of suchtravelling wave tubes, and that the heat thus produced in the delaylines actually limits the power such tubes may produce. This heatingefiect results principally, as is well known, from the absorption ofelectrons by the delay line.

This heating problem is particularly grave in the case ofbackward waveoscillators, such as described in the copending application, Serial No.281,347, filed on April 9, 1952, by B. Epsztein, now Patent No.2,932,760, in which the elements of the delay line adjacent the cath-'ode,=that is, the elements of the delay line which are exposed to theaction of the electron beam having maximum intensity, are subjected to amaximum ultra-high frequency field, and, consequently, to the maximumelectronic bombardment. This is so since in such backward waveoscillators the energy producing the ultra-high frequency field travelsin a backward direction, i.e., opposite the movement of the beam and isconstantly amplified in this backward direction by interaction with thebeam whereby the ultra-high frequency field reaches maximum proportionsnear the end thereof adjacent the cathode where ordinarily. the outputcoupling is also located.

, The present invention has for its object an arrangement whichlreducesthe resultant heating in such tubes.

Accordingly, it is an object of the present invention to provide a tubewith. an electron beam which propagates parallel to the delay line, thedelay line. including a portion, on the inside of which oneof the.dimensions of the section of the delay elements decreases from oneelement to the next elementin the direction of'the beam.

Theory. and experience have proved that the-organization of the electronbeam with a View tointeraction with the ultra-high frequency field ofthedelay line may be; effected in a more favorable manner if, in theinitial part-of thebeam, i.e., in the immediate neighborhood of thecathode,;the energy utilized for that .purposeis nottoo considerable ornot too large.

Under these conditions, the utilization of the .delay line ac'co'rdingtothe presentinventionpermits an improve- 1A further object of the presentinvention is the provi I sionf of 'anf arrangement in which' theamounti'offheat produced, which is ordina" y ,lirriitative'of theamountof po "er" output possiblefby' the tube, is effctivelyidei creased bytthe, arrangement of the delay line in i m ate ,Pr t u rlitw such a vUnited States Patent 2 i.e., a greater output with a tube of givenphysical dimen sion.

A still further object of the present invention resides in theconstruction of thedelay line which facilitates grouping of the electronbeam.

Still another object of the present invention is to provide a travellingwave tube which has a particularly favorable arrangement of the variousparts as regards production of heat due to absorption of electrons bythe delay line.

These and other objects, features and advantages of the presentinvention will become more obvious from the following description whentaken in connection with the accompanying drawing which shows, forpurposes of illustration only, two..preferred embodiments in accordancewith thepresent invention and wherein:

Figure 1 is a cross sectional view of a travelling wave tube havingcrossed electric and magnetic fields taken along line 11 of Figure 2; v

Figure 2 is a cross-sectional view in Figure 1 taken along line 2-2 ofFigure 1; and

Figure 3; illustrates anembodiment of the delay line with cylindricalbars which may be used in a tube according to the present invention.

The travelling wave tube in accordance with the present inventioncomprises the usual elements of such tubes such as, inter alia, acathode, a heater for the cathode, a collector, focusing means, a delayline and 'an electrode having a conductive surface essentially parallelto the delay line; potentials are applied to the respective elements toprovide an electric field which extends from the electrode to the delayline and a magnetic field'is also provided which has lines of fluxessentially perpendicular to the electric field and which is obtained inany suitable manner as, for example, by means of pole pieces arrangedcoaxially with the axis of the tube. The present'invention resides inthe particular configuration of the delay line, in which at least onedimension of each element forming the delay line varies in apredetermined manner. I

More particularly, the oscillator according to the present invention isprovided with a delay line in which one of the transverse dimensions ofthe elements thereof decreases progressively fromone element to theother, beginning with the end of the line adjacent the cathode andextending over a predetermined portion thereof, the

distance between the sole and thedelay line remaining essentiallyconstant throughout.

Referring now more particularly to the drawing,

wherein like reference numerals are used throughout the various views todesignate like parts, and more particularly to Figures 1 and 2 thereof,reference numeral 1 designates a metallic 'e'nvelope of the travellingwave tube. The travelling" wave tube itself may be of the type asdescribed in the aforementioned copending application by B. Epszteinwhich relates to a backward wave oscillator. L v

The cylindrical metallic envelope 1 is closed by ,two cover members 2and 3. p A cathode 10 heated by a'filament 11, and an electronic-opticalfocusing means of suitable construction-are disposed within the'tube.The electronic-optical focusing means may, for example, in-

clude a so-called, Wehnelt electrode 12 and, an intermediate electrode13which faces the cathode 10. ,.API'9 tures 4 and 5 are prpvidedinthecylinder ,1 which permit, respectively, passage therethrough of anoutput conductor] adapted to transfer the energy produced in the tubeand of a-plurality of conductors orrl'ead-in wires '-intended toapplydiiferent potential to the various electrodes of the tubes from anysuitable conventional power supply" i(not shown) 'I wo of these lead-inwires are of the tube illustrated I earn Figure 1 by referencefnumeralsQ"and .9,; 1 v

A delay line of the interdigital type 14, with a constant pitch, andwhich is composed of two combs 19- and 20, is fixed to the interior ofthe cylindrical envelope 1 as it is shown in Figure 2. The internalconducting surface of the envelope 1 plays the role of ceiling or basefor the delay line. The element 15 of the delay line which is locatednearest the cathode 10 is coupled to the output conductor 7. All of theelements of the delay line which are included between the elements 16and 17 inclusively are covered with any suitable attenuating means,although it is understood thatattenuation of these elements of the delayline may also be obtainedin any other suitable manner, as for example,by suitable attenuating'means either internal or external.

An electrodeZl, commonly referred 'to as sole, of which thecharacteristic form is shown in Figure 2-, is socured to and supportedby three insulating sleeves 22, 23 and 24 which are made of ceramicmaterial and which in turn are secured in any suitable manner, forexample,

soldered or brazed, to the cover member 2 of the tube. A collector 25 isfixed to the cylindrical envelope 1 at the end of the portion of thedelay line provided with the attenuating means.

According to the manner "of realization of the present invention asillustrated in the drawing, the height of the fingers of the delay line,that is, the dimension of the fingers in the direction perpendicular tothe ceiling or base of the line, decreases regularly over a substantialportion of the line from one finger to the next beginning with thefinger 15 located at the end i of the delay line adjacent the cathode upto the finger 28 which is equidistant of the two ends of the delay line.The fingers of the delay line included between the finger "28rand thefinger 1'7 have been illustrated as being provided with heights whichagain increase regularly.

in effect, the fingers of the delay line have been cut off in such amanner as to include the volume delimited by two cylinders havingparallel axes 26 and 27. The surfaces of the sole and of the delay linefacing each other being essentially parallel, it follows that the activepart of the sole 21 must, therefore, be constituted by the 'extern-a1surface of a cylinder having an axis 26, whereas the internal surface ofthercrown constituting the enyelope 1 of the tube is formed by acylinder with an axis 27. H

inthe embodiment illustrated in Figures 1 and '2, elemerits of the delayline are generally equi-distant from each other and the distance of thedelay line from the ceiling formed by the'internal surface of thecylindrical envelope 1 is the same from one end of the line to theother. -However, this-.isnot absolutelyinecessary, and the internalsurface of the envelope 1 may also be a V pylinder having an axis26. Inthat case, the spacing or distance of the delay line to theceiling orbase thereof formed by the internal conductive surface of the envelope 1would be greater at the place ofthe finger28 than at the place offingers 1 5 and i1.

. A suitable permanent magnet or electromagnet' of I which the polepieces are schematically indicated by reference numeral 2h and 34} inFigure}, provides a magnetic field having flux lines essentially:parallel to the axes 2'6 and 27. A radial electric field in theinteraction space between the delay line 14 and the sole 21 is obtainedby the difference in potential between. these two electrodes as appliedthereto frorna source of power by means. of

- later has been fully described inflthe' aforementionedcopendin'g'application. 1 7

As described therein," itis based onfthe interaction, in

wave induced in the delay line and propagated along the latter.

For a tube of this type, provided with an interdigital delay line,calculations made by the applicant have indicated that the couplingresistance is given by the following equation:

In this expression:

K is a constant which depends on the units.

I is the constant of propagation of the wave in interaction.

I1 is the distance between two consecutive fingers.

p is the pitch of the line.

d is the distance between the sole and the delay line.

I: is the height of the fingers D is the distance of the delay line tothe ceiling formed by the inner surface of the crown.

x is the distance between the sole and the beam.

-F or the normal values of these quantities, the coupling resistance isapproximately inversely proportional to the height of the fingers.

On the other hand, experience has indicated that the output of the tubeincreases considerably if the height of the fingers is decreased.Therefore, it is of interest to reduce the height thereof, but in thatcase, the capacity for dissipation of the heat also decreases with suchreduction.

According to the present invention, by increasing the height of thefirst few fingers of the delay line, on the one hand, the couplingresistance is decreased and therewith the amplitude or magnitude of thehigh frequency field at the entrance of the interaction spacewhile, onthe other, thecapacity for dissipation of these fingers is increased.The decrease of the high frequency field has as result that theelectrons of the beam which are absorbed thereat by the delay line aredistributed over. a larger surface, and that therewith the maximumdissipationvis further, increased. In addition thereto, the interactionis not established with its maximum intensity directly at the beginningof the interaction space but very progressively, which improves themechanics of the electronic grouping. Furthermore, it follows therefromthat the maximum dissipation isvery considerably increased withoutdecreasing the output in an appreciable manner; for the conditions ofoperation are thereby regularized, The inventor has experimentally notedthat the power provided by the tube of Figures'l and 2 is superior by atleast fifty per- 1 cent to that of a tube having analogouscharacteristics butprovided with a classical delay line, i.e., with adelay line ofthe prior art as-discussed hereinabove.

The delay line of the tube according to Figures 1 and 2 is particularlyeasy -to realizeinpractice. Eacho'f the combs is obtained by millin'g, ahollow metallic cylinder of which, to start with, the external andinternal surfaces are cylinders with parallel axes non-coaxial to eachother so that these'cylinders are eccentric with respect to each other.

- Figure 3 shows a different embodiment of a delay the interactionspiicebetween the sole-21 and the delay which may 'bensed in a tubeaccording to the present in vention. This embodiment relates to any typeof a line with bars, such as'a ladder-type delay line, a comb-typedelayline, or any other suitable type of delay line. delayhne isillustrated .in EigureB of the drawing in cross sectionin a planeperpendicularto the bars thereofQl t In. Figure 3; the sole, forexample, of, a linear tube which utilizes thelineis designated byreference nunieral '80, the ceiling or base of the line by referencenumeral 81, and the cross section of some of the successive bars byreference numerals 82 through 87. Depending on the particular case,these bars of the delay line may be supported at one of the ends thereofor at both ends thereof by means of lateral walls (not shown) which arerigidly secured to ceiling or base 81. According to the presentinvention, the diameter of these bars decreases regularly over asubstantial portion of the line from the bar 82 to the bar 87. In anoscillator which utilizes such a delay line, the output conductor wouldbe fixed to bar 82.

The rules which must beobserved for purposes of realization of a tubeaccording to the present invention which utilizes a delay line havingcylindrical bars are as follows: I

(1) The ratio of the capacities of each bar with respect to the ceilingor base formed by a conductive surface (and with respect to thefollowing bar is essentially constant; and

(2) The surface of the sole is parallel to the surface which is tangentto the bars on the side thereof nearest the sole.

It is understood, of course, that the present invention is not limitedto the embodiments illustrated and described herein which have beenillustrated only for exemplary purposes. In particular, the delay lineof the type represented in Figures 1 and 2 may be used in a linear tube,and particularly in a linear oscillator, and the type of the delay lineof Figure 3 may be used with a cylindrical tube, and more particularlyin a cylindrical oscillator. Moreover, the present invention is alsoapplicable to travelling wave tube amplifiers with suitable input andoutput means whereby the attenuating means may be omitted. In additionthereto, the ceiling or base of the delay line may be formed by aseparate conductive element other than the envelope, while any suitableattenuating means may be used with the respective elements of the delayline.

Thus, it is clear that the present invention is susceptible of manychanges and modifications within the spirit of the present invention andI intend to cover all such changes and modifications as encompassed bythe appended claims.

I claim:

1. A backward wave travelling wave tube comprising a constant pitchdelay line constituted by a chain of parallel conductive bars, a soleelectrode having a conductive surface opposite said delay line andbounding therewith an electron and wave interaction space, an electronbeam source near one end of said interaction space, and means forestablishing in said interaction space crossed electric and magneticfields for propagating said beam therethrough, the dimension of thecross section of said bars in the direction perpendicular to said solesurface gradually decreasing from one bar to the next over a firstportion of the delay line away from said source,

, and all of said elements being spaced apart from said sole electrodeby a constant interval.

2. A backward wave travelling wave tube according to claim 1, whereinsaid dimension increases away from said source beyond the end of saidfirst portion of said delay line. A

3. A backward wave travelling wave tube comprising a constant pitchinterdigital delay line made up of intercalated metallic fingers, a soleelectrode having a conductive surf-ace opposite said delay line andbounding therewith an electron and wave interaction space, an.

said s'ole electrode by a constant intervaln apart from '4. A backwardwave travelling wave tube according to claim 3, wherein said dimensionincreases awayfromsaid source beyondthe end of said first portion ofsaid delay line.' J H v p 5. A backward wave; travelling wave tubecomprising a constant pitch delay line constituted by a chain. ofparallel cylindrical bars, a sole electrode having a conductive surfaceopposite said delay line'and bounding therewith an electron and waveinteraction space, an electron beam source near one end of saidinteraction spacegandmeans for establishing in said interaction spacecrossed electric and magnetic fields for propagating said beamtherethrough, the diameters of said bars gradually decreasing from onebar to the next over a first portion of the delay line away from saidsource, and all of said bars being spaced apart from said sole electrodeby a constant interval.

6. A backward wave travelling wave tube according to claim 5, whereinsaid diameter increases away from said source beyond the end of saidfirst portion of said delay line.

7. A backward wave travelling wave tube according to claim 2, whereinsaid delay line is circularly curved and comprised between twocylindrical surfaces having noncoincident parallel axes.

8. A backward wave travelling wave tube comprising a constant pitchdelay line constituted by a chain of generally equi-distant parallelconductive bars, a sole electrode having a conductive surface oppositesaid delay line and bounding therewith an electron and waveinteraction-space, an electron beam source near one end of 9. In amicrowave oscillator of the type utilizing interaction between anelectron beam and an electromagnetic wave propagating along aretardation line in the direction opposite to the direction of flow ofsaid electron beam, said oscillator including an electron source andhaving an output coupling means disposed adjacent said source, theimprovement comprising a retardation line having a periodic structurepositioned adjacent the path traversed by said electron beam, saidretardation line having its coupling resistance gradually increased invalue over a substantial portion of said line in the direction of flowof said electron beam from a relatively low value in the vicinity ofsaid source to a relatively high value at a region along said linesubstantially removed from said source. V

10.-In a microwave oscillator of the type utilizing interaction betweenan electron beam and an electromagnetic wave propagating along aretardation line in the direction opposite to the flow of said electronbeam, said oscillator including an electron source and having an outputcoupling means disposed adjacent said source, the

improvement comprising a retardation line having peri-.

5 'pling resistance of said line to gradually increase in value saidsource.

crossed electric and magnetic fields for propagating said beamtherethrough, the dimension of a cross section of said fingers in thedirection perpendicular to said sole I surface gradually decreasing fromone finger to the neat over a first portion of the delay line away fromsaid over a substantial portion of said line in the direction of flow ofsaid electron beam from a relatively low value in the vicinity of saidsource to a relatively high value at a region along said linesubstantially removed from 11. A microwave oscillator comprising anelectron source and a collector electrode defining therebetween a pathof electron flow,,'a' delay'line'having periodically spaced digitspositioned along said path in which there is travel, the end of saidline remote from said electron source being terminated by digits ofincreased size, and attenuation means inserted in said line at saidremote end for absorbing wave energy.

References Cited in the fiie of this patent UNITED STATES PATENTS2,643,353 Dewey June 23, 1953 2,681,427 Brown et a1. June 15, 1954Warnecke et a1 Aug. 3 1, 1954 Pierce May 10, 1955 Peter Oct. 4, 1955Lerbs Jan. 10, 1956 Field Aug. 21, 1956 Miller et a1. Oct. 22, 1957Munushian Sept. 2, 1958 Birdsall Sept. 9. 1958 FOREIGN PATENTS FranceMay 30, 1950 Germany June 13, 1952 France -a-'--- Mar. 10, 1954

